Hi Allen,
Consider this the blind leading the blind.
Could this post-op pressurization be a due to a defective fuel pressure regulator, or one that is inappropriately sized or chosen for this particular application?
I can't picture anything the carbs could do to pressurize the fuel line after the engine stops. They'd almost have to have tiny pumps in the float valve area, so I'd suspect something about the behavior of the regulator instead. Then again, with Webers anything might be possible...
If this makes sense, it'll cost you a doughnut!
Tom